Fourier transform mass spectrometry (FT-MS) is generally understood to mean ion cyclotron resonance mass spectrometry (ICR-MS) because it records the mass-specific cyclotron motions of the ions as image currents in detection electrodes, and uses a Fourier transformation to convert the detected currents into a spectrum of the cyclotron frequencies. The spectrum of cyclotron frequencies is then converted into a mass spectrum by a mathematical transformation. Calibration constants are incorporated into the transformation function to take into account distortions to the frequency spectra caused, for example, by superimposed magnetron motions.
It has since emerged, however, that there are a range of mass spectrometric principles that allow mass-specific oscillations of ions to be used to compile mass spectra. These principles are distinguished by the fact that ions can be stored in specific cloud formations in two spatial directions by radial forces in a plane and that the ion clouds oscillate freely in a direction perpendicular to these two spatial directions in a potential which is as harmonic as possible. The radial forces that store the ion clouds can be magnetic fields, RF-generated pseudopotentials or radial electrostatic fields between central electrodes and outer shell electrodes.
In contrast to ICR mass spectrometers, these mass spectrometers do not detect an orbiting cyclotron motion of the ion clouds, but a backward and forward oscillating motion in the harmonic potential. If the radial forces are the same in all cross-sections along the direction of oscillation, ions of different masses oscillate as coherent ion clouds with different forms and different frequencies. The oscillations of the ion clouds can be measured in the form of induced image currents by suitably mounted detection electrodes. A Fourier analysis of these image currents produces the spectrum of the oscillation frequencies which occur in the mixture of oscillating ion clouds.
As is known, a harmonic potential is characterized by the fact that it creates a field which drives the ions deflected from the center back to the center again with a force proportional to the separation. This condition is fulfilled when the potential has a minimum in a center and increases as a parabola outside the center in the direction of the oscillation.
This new class of mass spectrometers includes the three-dimensional RF quadrupole ion traps operated with image current detectors, which are described in U.S. Pat. No. 5,625,186.
Another known embodiment uses a stack of plates to generate a three-dimensional quadrupole field in which ions can oscillate, see for example U.S. Pat. No. 5,283,436.
This class of mass spectrometer also includes the mass spectrometers manufactured by ThermoFisher and known by the trade name Orbitrap™ spectrometers, in which ions orbit in an electric radial field, on the one hand, and oscillate in an electric potential well in a direction perpendicular to this, on the other hand. The superimposed potentials are generated by two electrodes, an interior spindle and an exterior barrel.
It is possible to design more mass spectrometers in this new class, however. The ions can, for example, be made to oscillate between two pole rods in linear RF quadrupole ion traps (FIG. 3), in which case image current detector electrodes can be inserted between the pole rods.
The three-dimensional ion trap shown for example in FIG. 1 can also be operated with DC potentials and confined in a very strong magnetic field, producing a parabolic potential between the end caps in which ions can oscillate. These oscillations are known as “trapping oscillations”. The electrostatic field in the interior forms a saddle and the magnetic field must be very strong to keep the ions on the ridge of the saddle. For this, the image current detectors do not have to be very small; the whole of the end caps can be used as image current detectors.
A similar saddle-shaped electrostatic potential profile can also be generated with the aid of ring diaphragms, as shown for example in FIG. 4, if suitably calculated potentials are applied across the individual rings. Here, as well, it is possible to generate harmonic oscillations of the ions in a strong magnetic field. The potential here can be set so that there is a zero potential across two ring diaphragms, and these electrodes can be used as image current detectors.
All these oscillations in the direction transverse to the plane of the radial storage field can be tracked in suitable image current detectors and examined by Fourier analyses to establish the ion oscillation frequencies they contain. The Fourier analysis is essentially carried out as a fast Fourier transformation (“FFT”) of the image currents from the time domain into the frequency domain.
The mass spectrometers of this new class, diverse as they are, will be collectively termed “oscillation mass spectrometers” here because they all analyze harmonic oscillations of the ions in a harmonic potential. There is, as yet, only one embodiment of these oscillation mass spectrometers on the market, namely the Orbitrap™ mass spectrometer sold by ThermoFisher.
These oscillation mass spectrometers usually require a good vacuum so that, during the measuring period, the harmonically oscillating ion clouds do not diverge diffusely as the result of a large number of collisions. Furthermore, they require good ion injection conditions so that the ions can be collected in a suitably shaped ion cloud. The characteristic feature of this new class of oscillation mass spectrometer is a high mass resolution in the order of R=m/Δm=100,000, where m is the mass and Δm the full width at half-maximum of the mass signal. They are therefore better suited for the analysis of larger organic molecules. These larger organic molecules are generally ionized by electrospray ionization. The electrospray ionization generates the ions by protonating the molecules of the substance being analyzed; as a rule, not only singly charged ions are generated, but also large numbers of multiply charged ions are generated by multiple protonation.
In mass spectrometry, it is not the mass of the analyzed ions which is determined, but the mass-to-charge ratio m/z, where m is the physical mass and z the number of elementary charges on the ions.